1. Field of the Invention
The present invention relates to an objective lens for use in video disk, audio disk, and optical memory apparatus and the like, and especially relates to an aspherical single lens having a numerical aperture (NA) of about 0.5 and corrected regarding aberrations on and near the optical axis.
2. Description of the Prior Art
Conventionally the objective lens for the optical memory and the similar applications has been composed of three, four or five spherical lenses as disclosed, for example, in Japanese Patent Publication No. 44,209/1977 and Japanese Laid-Open Patent Application No. 148,143/1977.
The objective lens useful for the above applications is required not only to have high image-forming performance but also to be small and light. The compact structure (small size and light weight) is essential for following the automatic focusing and tracking control at high speed. The prior art objective lens composed of a spherical lens group comprising three to five component lenses is too heavy to satisfy the above requirement for compact construction. Furthermore, since it is a composite lens, it needs a very laborious and time-consuming produce for assembly and adjustment of the optical axis and lens-to-lens distances, which increases the manufacturing cost of the lens very much.
In order to solve the problem it has recently been proposed to use an aspherical single lens as the objective lens for optical memory. Examples of such aspherical single lens are those proposed in Japanese Laid-Open Patent Application Nos. 64,714/1982, 201,210/1982, 17,409/1983, and 68,711/1983 and U.S. Pat. No. 4,449,792. These prior art aspherical single lenses may be classified into two groups, one of which is of the type having two aspherical surfaces and the other of which is of the type having only one aspherical surface. The first type of aspherical lens whose surfaces are both aspherical is naturally more difficult to make than the second type which is aspherical on one side only. Therefore it is desirable to use the second type of aspherical single lens.
However, the single lenses having only one aspherical surface proposed by the above-mentioned prior inventions are unsatisfactory as the objective lens for optical memory in view of image-forming performance as well as compactness.